LCD devices have gradually replaced cathode ray tube (CRT) display devices, and have been widely used in various electronic devices such as high definition television (HDTV), desktop computer, personal digital assistant (PDA), notebook computer, mobile phone, digital camera, and so on, due to characteristics of thin thickness, low power consumption and low radiation etc.
An LCD device usually includes a color filter (CF) substrate, a thin film transistor (TFT) substrate, and a liquid crystal layer sandwiched between the CF substrate and the TFT substrate. A plurality of scanning lines, a plurality of data lines, a plurality of pixel electrodes, a common electrode and a plurality of TFTs are disposed on the TFT substrate. The scanning lines and the data lines are intersected with each other to define a plurality of pixel units. Each pixel unit includes a TFT and a pixel electrode formed on the pixel unit, wherein a gate electrode of the TFT is connected to a corresponding scanning line, a source electrode of the TFT is connected to a corresponding data line, and a drain electrode of the TFT is connected to the pixel electrode. When the scanning line receives a scanning signal outputted from a gate driving circuit, the TFT is turned on, and the data line receives a data signal which is a voltage outputted from a source driving circuit, and the pixel electrode is charged by the voltage of the data signal via the turned-on TFT. Therefore, a voltage difference between the pixel electrode and the common electrode drives the liquid crystal molecules in the liquid crystal layer to change arrangement, and accordingly light passed through the pixel unit is changed. After the light passes through a pair of polarizers of the LCD device, the change of the light can be presented as a change of brightness. Thus, the change of brightness of the LCD device can be controlled by controlling the voltage of the pixel electrode, so as to achieve the purpose of displaying images on the LCD device.
In the TFT, a parasitic capacitance Cgd is inevitably existed between the gate electrode and the drain electrode of the TFT. During the driving of the LCD device, at the moment when the TFT turns on or off, the voltage of the pixel electrode will change sharply and dramatically due to the sharp change of the voltage on the scanning line and the influence of the parasitic capacitance Cgd between the gate electrode and the drain electrode of the TFT. Specifically, at the moment when the TFT turns on, due to the sharp change of the voltage (i.e., from low voltage to high voltage) on the scanning line and the influence of the parasitic capacitance Cgd, an upward feed-through voltage is generated on the pixel electrode. Since the data line is charging the pixel electrode via the turned-on TFT, the pixel electrode still can be charged to a desired voltage outputted by the date line. However, at the moment when the TFT turns off, due to the sharp change of the voltage (i.e., from high voltage to low voltage) on the scanning line and the influence of the parasitic capacitance Cgd, a downward feed-through voltage is generated on the pixel electrode. After the TFT is turned off, the data line stops to charge the pixel electrode, and the voltage on the pixel electrode drops by a value of the feed-through voltage due to the influence of the parasitic capacitance Cgd. That is, the voltage on the pixel electrode will be smaller by a value of the feed-through voltage than the charged voltage of the pixel electrode as previously charged when the TFT is turned on. Therefore, the accuracy of the voltage intended to be charged on the pixel electrode is affected, and accordingly the accuracy of a gray scale displayed on the LCD device is affected, the display quality of the LCD device is thus deteriorated.
In addition, due to positive and negative impurity ions being inevitably existed in the liquid crystal molecules of the liquid crystal layer, these impurity ions are easily adsorbed on the pixel electrode and the common electrode under a driving electric field generated between the pixel electrode and the common electrode for driving the liquid crystal molecules. Therefore, an image sticking problem will be resulted on the screen of the LCD device as a result of these impurity ions, and the display quality of the LCD device is further deteriorated.
At present, a common voltage on the common electrode is usually offset in order to compensate and resolve the problem caused by the parasitic capacitance Cgd of the TFT. That is, when the common electrode has a DC (direct current) common voltage, the DC common voltage is not zero, but is offset by a value such as 3V. When the common electrode has an AC (alternating current) common voltage, a center value of the AC common voltage is not zero, but is offset by a value such as 3V. Many methods such as improving the purity of the liquid crystal molecules and improving the cleanliness of circumstance in manufacturing the LCD device are used to resolve the problem caused by the impurity ions in the liquid crystal molecules of the liquid crystal layer. However, the methods mentioned above need high cost and long adjustment period.